Backstay tension & the Selden method...

[RobbieW]

Using the method described in the Selden Rig Tuning guide for measuring increase in length over 2 metres as a way of getting the base tension right, 15% load they say. I understand the scenario for cap shrouds as the sides are symetrical, so winding up the turnbuckle to get 1.5 mm increase on one side then wind up the other to match so that the increase is now 3mm on the measured side makes sense. The backstay/forestay are not symetrical however, so assuming I have the measure on the backstay am I still trying to increase the gap to 3mm ?

 

[Quandary]

I think you need to read the guide a bit more, it is unusual for the tension in the backstay to match the forestay but how tight it needs to be depends on the configuration of your rig. Seldens manual addresses most of the common configurations. A tight (straight) forestay is desirable but it is not usually entirely dependent on backstay tension. Even with a mast head rig there is often some scope for adjusting backstay tension to adjust mast shape.

 

[RobbieW]

I have a masthead rig with a single inline spreader and both fore and aft lowers, page 37 in my copy of the guide - Rival 41c. I've set a little pre-bend in the mast with the forward lowers, there is also no backstay adjuster to change the shape after set up. I've got to the point where I'd need a bigger lever if I'm to get 3mm in the backstay, that bothers me hence the question.